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In the mean time, I found this on Trip Advisor:
"Do the tour!
We booked this historical tour with Royden for my husband's birthday. Luckily there was just the two of us and we were able to have a great 2 hours walking around this beautiful hotel and learning about the history of the railway and how the hotel came to be... The cost was £20 each and we had a complimentary cup of tea and a cake at the end which was lovely. If you love the railway or architecture then definitely take a look!"
So it seems as though you do get a cup of tea!
As we would be leaving London of the Eurostar, this could be the ideal end to a short break in London. I'm still tempted instead of the steep price. If we do do it, I'll report back (in November).